Thoughts from the dark side....
I don't like to answer "how-to" posts, as this kind of stuff should be left up to the instructors and teachers, but maybe I can just give some food for thought. This is how I have been taught, and can vouch for its effectiveness.
Sute ukemi (breakfalls) and zenpo kaiten (forward rolling) are taught as the same physiological process, the only difference being the connection point. In sute ukemi it is fixed, causing uke to rotate around it; in zenpo kaiten it is translatory, causing uke to move with it. This being the case, we practice all of our "breakfalls" (sute ukemi) by doing forward rolls first. Most anyone can practice both forms of ukemi during "kokyu nage", which are normally just softer variations of what would be called "te waza" (hand techniques) in Kodokan Judo. So in answer to the original question about learning to fall correctly for "scary" throws, koshi or otherwise, do a lot of forward rolling until you are comfortable with the movement pattern, then have the tori "tighten up" the focus (I don't mean to say get stronger or meaner). You will be doing sute ukemi (breakfalls) with ease, especially once your body adjusts to the tension-relaxation pattern of falling.
The different posts in this thread raised a somewhat rhetorical question for me though. If we regard kokyu nage as some form of hand technique, that is, the connection and decisive action is conducted through the hands, why do we change the rules for koshi waza? All this talk of lifting and lower body strength, special drills on where to put your arms, etc. Most anyone can deadlift more than they can benchpress, why is it easier to throw someone with the hands, as in most "kokyu nage", than it is in koshinage?
I think the answer lies in nage's perception of power within any technique. Like Jun said earlier about footsweeps, manipulation of posture and balance until the uke is touched at a "focal fulcrum" like the feet, knees, hips, shoulders or head results in the best throws. Koshi waza are not hip throws because we use the legs and hips to pick up the uke and throw them down; koshi waza are hip throws because the focal point of the throw, the fulcrum if you will, happens around the hips.
In theory, not only could we teach someone's grandma to fall for a good ogoshi, we could also teach Grandma to throw us just as hard as we threw her. At least that was the idea of the individual who made the term "koshi waza" famous.